1. Field of the Invention
The present invention relates to a magnetic resonance imaging apparatus for performing multi-slice imaging based on a variable TR scanning scheme.
2. Description of the Related Art
Recently, a variable TR scanning scheme is developed in order to shorten the scanning time. According to this scheme, echo data is acquired not by setting a constant repetition time TR regardless of the value of the spatial frequency in the phase-encoding direction, but by changing the repetition time TR in accordance with the value of the spatial frequency component. Generally, the repetition time TR affects the contrast of an image. The contrast largely depends on the low frequency component of the image. A high frequency component does not much affect the contrast but rather affects the resolution. Therefore, when the spatial frequency in the phase-encoding direction is almost 0, the repetition time TR is set to a sufficiently long time for obtaining a contrast required for an image, and when the absolute value of the spatial frequency is large, thus not much affecting the contrast, the repetition time TR is set shorter than that for the spatial frequency of almost 0 in the phase-encoding direction. In this manner, according to the variable TR scanning scheme, the scanning time can be shortened.
When, however, the variable TR scanning scheme is applied to multi-slice imaging wherein a plurality of slices are excited within the repetition time TR, an inconvenience is caused. More specifically, in each encoding step, a time required for acquiring echo data per slice is predetermined, and the number of slices to be scanned within the repetition time TR is equal to (TR)/(time required for acquiring echo data per slice). Hence, the number of slices at the high-frequency component acquired by shortening the repetition time is smaller than the number of slices at the low-frequency component acquired by prolonging the repetition time.
Regarding a slice whose high-frequency component echo data are not obtained, an image of such a slice cannot be reconstructed by acquiring only its low-frequency component echo data. Therefore, in variable TR scanning multi-slice imaging, the number of slices is limited by the number of slices that can be acquired within the minimum repetition time TR. More specifically, in an encoding step having a repetition time TR longer than the minimum repetition time TR, although slice data corresponding to a larger number of slices than the number of slices that can be acquired within the minimum repetition time TR can be acquired, data acquisition is not performed in this step, and spin recovery is merely waited for in order to increase the contrast. For example, assume that only data corresponding to three slices can be acquired in a high-frequency encoding step. In this case, even if slice data corresponding to a larger number of slices can be acquired in a low-frequency encoding step having a long repetition time, as the high-frequency component data concerning resolution are not acquired, echo data corresponding to only three slices are acquired.
In this manner, according to the conventional magnetic resonance imaging apparatus for performing variable TR scanning, although the scanning time required for obtaining an image having a desired contrast can be shortened, the number of slices to be scanned within the repetition time TR is smaller than that obtained before the scanning time is shortened by shortening the repetition time TR.